As the world focuses on the World Cup, which opens in Brazil in less than a fortnight, many Brazilians are wrestling with painful discoveries about the military dictatorship that ruled the country from 1964 to 1985. The BBC has found evidence that the UK actively collaborated with the generals - and trained them in sophisticated interrogation techniques.

One of the most feared torturers, Col Paulo Malhaes, gave 20 hours of testimony. He arrived in a wheelchair looking frail. He then confessed to killing and mutilating his victims. He also expressed great admiration for psychological torture which, he felt, was more effective than brute force, especially when it came to turning a left-wing militant into an infiltrator.

Prof Glaucio Soares interviewed more than a dozen of Brazil's top generals back in the 1990s. Several of them told him they sent officers to Germany, France, Panama and the US to learn about interrogation but they praised the UK as having the best method.

"The Americans teach, but the English are the masters in teaching how to wrench confessions under pressure, by torture, in all ways. England is the model of democracy. They give courses for their friends," he was told by Gen Ivan de Souza Mendes - an interview recounted in the book Years of Lead which he co-authored with two other Brazilian academics.

Gen Aoyr Fiuza de Castro said the British recommend interrogating a prisoner when he was naked as it left him anguished and depressed, "a state favourable to the interrogator".